Trisubstituted piperazin-2,5-diones

ABSTRACT

Disclosed is a process for preparation of an amino acid, such as phenylalanine, with a high degree of optical purity. The process makes use of the same amino acid as a chiral template.

This is a divisional of copending application Ser. No. 07/406,995 filedon Sep. 14, 1989, now U.S. Pat. No. 4,992,552 which is acontinuation-in-part of Ser. No. 07/239,492, filed Aug. 31, 1988 nowabandoned.

FIELD OF INVENTION

The present invention concerns a process for synthesis of an amino acidwherein the same amino acid serves as its own chiral template.

BACKGROUND OF THE INVENTION

Amino acids are the building blocks of proteins and are thereforeessential for life itself. Amino acids can be either of the type foundin proteins of biological sources (so called naturally occurring) or canbe of other types that are synthesized chemically (so called syntheticamino acids). Amino acids of either the naturally occurring type or thesynthetic type have a multitude of uses including use as a nutritionalsource or intermediate therefor and use as building blocks for variousbiologically active peptides and proteins (e.g. see for example, C. Y.Bowers, et. al., European Patent Application WO 87/06835).

Alpha-amino acids typically have one asymmetric carbon atom andtherefore can be either in the L or D form. In most cases the L form isthe form found in proteins of biological sources. For variousapplications it is desirable to have only one optically active form ofthe amino acid (rather than the other form or a racemic mixture).Therefore processes for production of an amino acid with a particulardegree of optical purity are highly desirable.

SUMMARY OF THE INVENTION

The present invention is directed to a process for preparation of asubstantially optically pure alpha-amino acid wherein the same aminoacid serves as its own chiral template. The process of the presentinvention employs a novel hydrogenation step wherein a cyclic compoundhaving an asymmetric carbon of either L or D configuration and anunsaturated carbon-carbon double bond is converted to a cyclic compoundwith two asymmetric carbon atoms in substantially the cis-form. Suchhydrogenation step shall be referred to herein as "the hydrogenationstep". More specifically, the hydrogenation step comprises

Contacting Compound I of the formula ##STR1## wherein R is hydrogen,hydroxy, alkyl, aryl, C₁ to C₁₀ substituted alkyl, C₁ to C₁₀ alkoxy, C₇to C₁₂ arylalkyl, C₇ to C₁₂ substituted arylalkyl, C₁ to C₁₀carboxyalkyl or C₁ to C₁₀ acyloxy, R' is the same as R; Z is anitrogen-protecting group or hydrogen; and Z' is the same or differentthan Z and is a nitrogen-protecting group or hydrogen; with hydrogen inthe presence of a suitable catalyst and suitable solvent to formcompound II of the formula ##STR2## wherein the diastereomeric purity ofcompound II is at least about 70%, preferably at least about 90% of thecis derivative, more preferably at least about 95% of the cisderivative.

As used herein, diastereomeric purity refers to the % cis isomer and canbe expressed mathematically as % diastereomeric purity

DETAILED DESCRIPTION OF THE INVENTION

The undulating lines (i.e. ^(])) connecting various substituents in theformulas appearing herein indicate bonds wherein the stereochemistry ofthe asymmetric carbon atom containing the bond (^(])) is independentlyin the D or L configuration. As used herein "D" or "L" configurationrefers to the chirality of the carbon atom adjacent to the carbonylcarbon (the alpha position); such carbon atom is hereinafter referred toas the alpha carbon atom in amino acids and amino acid derivatives. Theterms D-configuration and L-configuration are commonly understood tothose experienced in the art (e.g., see, pp. 80-83, A. L. Lehninger,Biochemistry, Second Edition, Worth Publishers, Inc., New York, NewYork, 1977).

It is to be understood that when one or more steps are performedconsecutively wherein a compound has two asymmetric alpha carbon atomsas indicated, such two asymmetric carbon atoms will be substantially inthe cis form. As used in this context "substantially" means that atleast about 70%, more preferably at least about 90% and most preferablyat least about 95% of the desired compound will be in the desired form.

As used herein the term "optical purity" refers to an amino acid orderivative thereof and can be expressed mathematically in percent as anabsolute value as ##EQU2##

The term "alkyl" means straight, branched or cyclic alkyl moieties of upto 20 carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl.Preferred alkyl groups are C₁ to C₁₀, more preferred are C₁ to C₆, andmost preferred are methyl, isopropyl, and isobutyl.

The term "C₁ to C₁₀ substituted alkyl" denotes the above C₁ to C₁₀ alkylgroups that are substituted by one to four halogen, hydroxy, amino, C₁to C₇ acyloxy, nitro, carboxyalkyl, carbamoyloxy, cyano, or C₁ to C₆alkoxy groups. The substituted alkyl groups may be substituted once orup to four times with the same or with different substituents. Preferredsubstituted alkyl are C₁ to C₆ substituted alkyl groups.

Examples of the above substituted alkyl groups include the cyanomethyl,nitromethyl, hydroxymethyl, trityloxymethyl, propionyloxymethyl,aminomethyl, carboxymethyl (i.e., --CH₂ --COOH), allyloxycarbonylmethyl,allyloxycarbonylaminomethyl, carbamoyloxymethyl, methoxymethyl,ethoxymethyl, 1-butoxymethyl, acetoxymethyl, chloromethyl, bromomethyl,iodomethyl, 6-hydroxyhexyl, 2-amino(iso-propyl), 2-carbamoyloxyethyl andthe like. A preferred group of examples within the above "C₁ to C₁₀substituted alkyl" group includes the substituted methyl group, in otherwords, a methyl group substituted by the same substituents as the "C₁ toC₁₀ substituted alkyl" group. Examples of the substituted methyl groupinclude groups such as hydroxymethyl, acetoxymethyl, carbamoyloxymethyl,carboxymethyl, chloromethyl, bromomethyl and iodomethyl.

The term "C₁ to C₁₀ alkoxy" as used herein denotes groups of the formulaOR⁷ wherein R⁷ is hydrogen or alkyl. Preferred alkoxy groups includesuch groups as methoxy, ethoxy, n-propoxy, isopropoxy, butoxy, and likegroups.

The term "C₁ to C₁₀ acyl" or "acyl" denotes groups of the formula##STR3## containing between 1 and 10 carbon atoms, wherein R³ ishydrogen, alkyl, aryl, substituted alkyl, arylalkyl, and substitutedarylalkyl. Examples of preferred C₁ to C₁₀ acyl groups are those whereinR³ is a C₁ to C₆ alkyl group such as methyl (Me), ethyl (Et), propyl(Pr) or butyl (bu).

The term "enol ester" refers to a compound of the formula ##STR4##wherein R³ is defined hereabove.

The term "C₁ to C₁₀ acyloxy" or "acyloxy" denotes groups of the formula##STR5## containing between 1 and 10 carbon atoms, wherein R³ is asdefined hereabove. Examples of preferred C₁ to C₁₀ acyloxy groupsinclude those wherein R³ is a C₁ to C₆ alkyl group such as methyl,ethyl, propyl, or butyl. Additional preferred acyloxy groups includethose wherein R³ is aryl or arylalkyl such as phenyl or benzyl.

The term "C₁ to C₁₀ carboxylalkyl" denotes groups of the formula##STR6## wherein R³ is as defined hereabove. Examples of preferred C₁ toC₁₀ carboxyalkyl groups are those wherein R³ is a C₁ to C₆ alkyl groupsuch as methyl, ethyl, propyl or butyl.

The term "aryl" refers to aromatic groups of 3 to 50 carbon atoms whichinclude heterocyclic rings and unsubstituted and substituted aryls. Themost preferred aryl is phenyl.

The term "substituted aryl" specifies an aryl group (preferred is aphenyl group) substituted with one to four moieties chosen from thegroup consisting of halogen, hydroxy, protected hydroxy, cyano, nitro,C₁ to C₄ alkyl, C₁ to C₄ alkoxy, carboxy, carboxymethyl, hydroxymethyl,aminomethyl, trifluoromethyl or N-(methylsulfonylamino). Phenyl shall bealternately referred to herein by the symbols "φ" or "Ph".

Examples of the term "substituted aryl" include mono-, di- or tri(halo)phenyl group such as 4-chlorophenyl, 2,6-dichlorophenyl,2,5-dichlorophenyl, 3,4-dichlorophenyl, 3-chlorophenyl, 3-bromophenyl,4-bromophenyl, 3,4-dibromophenyl, 3-chloro-4-fluorophenyl,2-fluorophenyl, and the like; a mono-, di , or tri(hydroxy)phenyl groupsuch as 4-hydroxyphenyl, 3-hydroxyphenyl, 2,4-dihydroxyphenyl, theprotected hydroxy derivative thereof, and the like, a nitrophenyl groupsuch as 3 or 4-nitrophenyl, a mono-, di- or tri(lower alkyl)phenyl groupsuch as 4-methylphenyl, 2,4-dimethylphenyl, 2-methylphenyl,4-(iso-propyl)phenyl, 4-ethylphenyl, 3-(n-propyl)phenyl, and the like; amono- di or tri(alkoxy)phenyl group, for example 2,6-dimethoxyphenyl,4-methoxyphenyl, 3-ethoxyphenyl, 3-(isopropoxy)phenyl,4-(t-butoxy)phenyl, 3,4,5-trimethylphenyl, 3-ethoxy-4-methoxyphenyl, andthe like; a mono- or dicarboxyphenyl group such as 4-carboxyphenyl; amono- or di(hydroxymethyl)phenyl such as 3,4-di(hydroxymethyl)phenyl; amono- or di(aminomethyl)phenyl such as 2-(aminomethyl)phenyl or a mono-or di (N-(methysulfonylamino))phenyl such as3-(N-(methylsulfonylamino))phenyl. Also, the term "substituted phenyl"represents disubstituted or trisubstituted phenyl groups wherein thesubstituents are different, for example, 3-methyl-4-hydroxyphenyl,3-chloro-4-hydroxyphenyl, 2-methoxy-4-bromophenyl,4-ethyl-2-hydroxyphenyl, 3-hydroxy-4-nitrophenyl,2-hydroxy-4-chlorophenyl and the like.

The terms "halo" and "halogen" refer to the fluoro, chloro, bromo oriodo groups.

The term C₇ to C₁₂ arylalkyl denotes a C₁ to C₆ alkyl group substitutedat any position by a aromatic ring. Examples of such a group includephenylmethyl (benzyl), 2-phenylethyl, 3-phenyl(n-propyl), 4-phenylhexyl,3-phenyl-(n-amyl), 3-phenyl-(sec-butyl), and the like. A preferred groupis the benzyl group.

The term "C₇ to C₁₂ substituted arylalkyl" denotes a C₇ to C₁₂ arylalkylgroup substituted on the C₁ to C₄ alkyl portion with one or two groupschosen from halogen, hydroxy, C₁ to C₇ acyloxy, nitro carboxy,carbamoyl, carbamoyloxy, cyano, N-(methylsulfonylamino) or C₁ to C₄alkoxy; and/or the aromatic group may be substituted with 1 or 2 groupschosen from halogen, hydroxy, nitro, C₁ to C₄ alkyl, C₁ to C₄ alkoxy,carboxy, carboxymethyl, hydroxymethyl, aminomethyl, or aN-(methylsulfonylamino) group. As before, when either the C₁ to C₄ alkylportion or the aromatic portion or both are disubstituted, thesubstituents can be the same or different.

Examples of the term "C₇ to C₁₂ substituted arylalkyl" include groupssuch as 2-phenyl-1-chloroethyl, 2-(4-methoxyphenyl)ethyl,2,6-dihydroxy-4-phenyl(n-hexyl), 5-cyano-3-methoxy-2-phenyl(n-pentyl),3-(2,6-dimethylphenyl)n-propyl, 4-chloro-3-aminobenzyl,6-(4-methoxyphenyl) -3-carboxy(n-hexyl), 5-(4-aminomethyl-phenyl)3-(amino)-(n-hexyl), and the like.

The term "heterocyclic ring" denotes optionally substitutedfive-membered or six-membered rings that have 1 to 4 heteroatoms, suchas oxygen, sulfur and/or nitrogen, in particular nitrogen, either aloneor in conjunction with sulfur or oxygen ring atoms. These five-memberedor six-membered rings may be fully unsaturated or partially unsaturatedwith fully unsaturated rings being preferred.

Furthermore, the above optionally substituted five-membered orsix-membered rings can optionally be fused to an aromatic 5-membered or6-membered ring system such as a pyridine or a triazole system, andpreferably to a benzene ring.

The following ring systems are examples of the heterocyclic (whethersubstituted or unsubstituted) radicals denoted by the term "heterocyclicring" and are nonlimiting: thienyl, furyl, pyrrolyl, imidazolyl,pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl,thiadiazolyl, oxadiazolyl, tetrazolyl, thiatriazolyl, oxatriazolyl,pyridyl, pyrimidyl, pyrazinyl, pyridazinyl, thiaziayl, oxazinyl,triazinyl, thiadiazinyl, oxadiazinyl, dithiazinyl, dioxazinyl,oxathiazinyl, tetrazinyl, thiatriazinyl, oxatriazinyl, dithiadiazinyl,imidazolinyl, oxhydropyrimidyl, tetrahydropyrimidyl,tetrazolo(1,5-b)pyridaziny; and purinyl, as well as benzo-fusedderivatives for example benzoxazolyl, benzthiazolyl, benzimidazoly andindolyl.

A preferred group of examples of the above heterocyclic rings are5-membered ring systems containing a sulfur or oxygen atom and/or one tothree nitrogen atoms. Examples of such preferred groups includethiazolyl, thiadiazolyl, and oxazolyl. A group of further preferredexamples of 5-membered ring systems with 2 to 4 nitrogen atoms includeimidazolyl, triazolyl, and tetrazolyl.

Further specific examples of the above heterocyclic ring systems are6-membered ring systems containing one to three nitrogen atoms. Suchexamples include pyridyl, pyrimidyl, triazinyl, pyridazinyl, andpyrazinyl.

The substituents for the optionally substituted heterocyclic ringsystems and further examples of the 5- and 6-membered ring systemsdiscussed above, are found in W. Durckheimer et at., U.S. Pat. No.4,278,793, incorporated herein by reference issued Jul. 14, 1981,columns 9 through 21 and columns 33 through 188 (examples of the term"heterocyclic ring" are included in the heterocyclic thiomethyl groupslisted under heading "A").

The more preferred aryls are phenyl, naphthyl, pyridyl, and indole; andthe most preferred aryl is phenyl.

The term "nitrogen-protecting group" as used in the specification andclaims refers to substituents of an amide or amino group commonlyemployed to block or protect the amino or amide functionality whilereacting other functional groups on the compound. Examples of suchnitrogen-protecting groups include but are not limited to acetyl, theformyl group, the trityl group, the phthalimido group, thetrichloracetyl group, the chloroacetyl, bromoacetyl, and iodoacetylgroups, urethane-type blocking groups such as benzyloxycarbonyl,4-phenylbenzyloxycarbonyl, 2-methylbenzyloxycarbonyl,4-methoxybenzyloxycarbonyl, 4-fluorobenzyloxycarbonyl,4-chlorobenzyloxycarbonyl, 3-chlorobenxyloxycarbonyl,2-chlorobenzyloxycarbonyl, 2,4-dichlorobenzyloxycarbonyl,4-bromobenzyloxycarbonyl, 3-bromobenzyloxycarbonyl,4-nitrobenzyloxycarbonyl, 4-cyanobenzyloxycarbonyl,1,1-diphenyleth-1-yloxycarbonyl, 1,1-diphenylprop-1-yloxcarbonyl,2-phenylprop-2-yloxycarbonyl, 2-(p-toluyl)prop-2-yloxycarbonyl,cyclopentanyloxycarbonyl, 1-methylcyclopentanyloxycarbonyl,cyclohexanyloxycarbonyl, 1-methylcyclohexanyloxycarbonyl,2-methylcyclohexanyloxycarbonyl, 2-(4-toluylsulfonyl)ethoxycarbonyl,2-(methylsulfonyl)-ethoxycarbonyl,2-(triphenylphosphino)-ethoxycarbonyl, 9-fluorenylmethoxycarbonyl("FMOC"), t-butoxycarbonyl ("BOC"), 4-acetoxybenzyloxycarbonyl,2,2,2-trichloroethoxycarbonyl, 2-ethynyl-2-propoxycarbonyl,cyclopropylmethoxycarbonyl, 4-(decyloxy)benzyloxycarbonyl,isobornyloxycarbonyl, 1-piperidyloxycarbonyl and the like; thebenzoylmethylsulfonyl group, the 2-(nitro)phenylsulfenyl group, thediphenylphosphine oxide group and like amino-protecting groups. Thespecies of nitrogen-protecting group employed is not critical so long asthe derivatized amide or amino group is stable to the conditions ofsubsequent reaction(s) which effect other positions of the molecule andcan be removed at the appropriate point without disrupting the importantfunctional group(s) of the subsequent product molecule(s). Preferrednitrogen-protecting groups are the allyloxycarbonyl, thebenzyloxycarbonyl, t-butoxycarbonyl, trityl, acetyl and substitutedacetyl groups. The most preferred nitrogen protecting groups are acetyl(Ac) and alpha-halo-acetyl. Similar amino-protecting groups used in thepeptide art are also embraced by the above term. Further examples ofgroups referred to by the above term are described by J. W. Barton,"Protective Groups in Organic Chemistry", J. G. W. McOmie, Ed., PlenumPress New York, N.Y., 1978 Chapter 2, and T. W. Greene, "ProtectiveGroups in Organic Synthesis", John Wiley and Sons, New York, N.Y. 1981,Chapter 7. The related term "Protected nitrogen" defines an amino oramide group substituted with a nitrogen-protecting group discussedabove.

A preferred process for the present invention involves several stepsstarting with one mole of a substantially optically pure amino acid andending with approximately two moles of the same amino acid having thesame optical configuration and substantially the same optical purity.The preferred amino acids for use in the present invention includephenylalanine, isoleucine, leucine, lysine, methionine, threonine,tryptophan, valine, alanine, aspartic acid, glutamic acid, arginine,asparagine, cysteine, glutamine, histidine, serine, and tyrosine.

The most preferred amino acid is phenylalanine, especiallyL-phenylalanine.

In the preferred compounds described herein R and R' are the same andare preferably aryl with phenyl and substituted phenyl being morepreferred, and phenyl being most preferred; generally for Compounds Iand II, Z' is preferably H. For Compounds I and II, Z is preferablyacyl, acyloxy, or hydrogen. For compounds I and II, Z is most preferablyacetyl or hydrogen.

Regarding the Q substituents of the compounds described hereinafter, Qis capable of being displaced by a nitrogen-containing nucleophile,preferably ammonia or a substituted mine. Many Q groups are oftenreferred to in the art as esters or active esters, and are commonly usedas activating groups in the peptide art. Example of Q groups aredescribed by M. Bodansky in "Active Esters in Peptide Synthesis," pp.105-196, The Peptides, I, E. Gross, J. Meienhofer, editors, AcademicPress, New York, New York, 1979. Except for compounds ZQ and Z¹ Qdescribed hereinafter, preferred Q groups include chloro, bromo, iodo,--SR⁴ or OR⁴ wherein R⁴ is H, alkyl, aryl, C₁ to C₁₀ substituted alkyl,C₇ to C₁₂ arylalkyl, C₇ to C₁₂ substituted arylalkyl, C₁ to C₁₀ acyl,and C₁ to C₁₀ acyloxy. Other Q substituents are nitrogen-containingcyclic moieties such as imidazole, succinimide, and phthalimide.

The first step ("Step 1") of the process of the present invention is aprocess for preparing a compound of the formula ##STR7## which comprisescontacting a compound of the formula ##STR8## or salt form thereof witheither (a) a compound of the formula ##STR9## in the presence of asuitable solvent and under acidic, neutral, or basic conditions andunder other conditions such that a compound is formed having thefollowing formula ##STR10## followed by reacting Compound VI with asuitable nucleophile of the formula

    HQ                                                         (VII)

in the presence of a suitable solvent under conditions such that anactual or formal loss of water occurs and Compound III is formed, or

(b) a suitable nucleophile of the formula

    HQ                                                         (VII)

in the presence of a suitable solvent, under conditions such that thereis formed a compound of the formula ##STR11## or salt form thereoffollowed by reacting compound VIII with compound V in a suitable solventunder conditions such that compound III is formed;

wherein Q is as defined hereinbefore; and Y is a group capable ofundergoing nucleophilic displacement most preferably with ammonia tointroduce nitrogen at the carbon atom substituted with Y, or is amino oran amino group substituted with one or two nitrogen-protecting groups,and Y' is a group capable of undergoing nucleophilic displacement. Y'groups are capable of undergoing nucleophilic displacement upon reactionwith compound IV or compound VIII. Examples of such groups include haloand alcohol leaving groups known in the art such as tosylates andmesylates. In the case where Y is amino or an amino group substitutedwith one or two nitrogen protecting groups, while Y may in some cases becapable of undergoing nucleophilic displacement, such a displacement isnot generally necessary to introduce nitrogen at the carbon atomsubstituted with Y.

Suitable solvents for Step 1 as well as for the steps that follow arethose solvents capable of solubilizing the reactants sufficiently enoughto allow the desired process step to proceed without significant adverseeffects. Suitable solvents for Step 1 include polar or nonpolar, proticor aprotic solvents such as C₁ to C₁₀ aliphatic or aromatic alcohols,e.g., methanol (MeOH), ethanol (EtOH) and isopropanol (iPrOH);dimethylformamide; tetrahydrofuran; water; C₁ -C₂₀ straight chain orbranched chain carboxylic acids or esters derived therefrom; toluene;methylene chloride; and mixtures thereof. As appreciated in the art,varying solvents and other process conditions may affect the processsteps. Routine experimentation may be required to determine desired oroptimal process conditions.

Preferred conditions for Step (1)(a) include reacting compound IV withcompound V under basic conditions and a reaction temperature of about-80° to 300° C., more preferred is -20°to 100° C.; and most preferred isabout 0° to 40° C. Preferred molar ratios of compound IV:compound V areabout 1:10 to 10:1; preferred is 1:3 to 3:1.

Preferred conditions for Step (1)(a) involve reacting compound VI withcompound VII to produce Compound III, and include use of a C₁ to C₁₀aliphatic or aromatic alcohol, more preferably the solvent is the sameas the nucleophile (i.e., compound VII). The most preferred solvents areMeOH, EtOH, iPrOH, and benzyl alcohol. This reaction is also preferablyperformed under acidic conditions and/or in the presence of adehydrating agent in the amount necessary to dehydrate the carboxylicacid moiety of compound VI. Such dehydrating agents include 4A molecularsieves, 3A molecular sieves, magnesium sulfate, dicyclohexylcarbodiimideand related carbodiimides, carbonyldiimidazole, thionyl chloride, andthe like. The preferred reaction temperatures for reacting compound VIwith compound VII are about the same as those for reacting compound IVwith compound V, except that in the former case, in acidic conditionswith alcohol solvents, higher temperatures are generally preferred.Molar ratios of compound VI:compound VII are preferably 1:1000 to 10:1;1:1 to 1:100 being more preferred.

Other preferred conditions for step 1(a) include reacting a compound ofthe formula IV with an alpha-halo-acetyl halide (V) wherein Y and Y' arechosen from the group consisting of chlorine, bromine, and iodine toproduce a compound of the formula VI. Alternatively, IV may be allowedto react with glycine or an appropriately protected and/or activatedform of glycine such that a glycine substituted dipeptide VI is formedwherein Y is either NH₂ or an appropriately substituted nitrogen atom.

Other preferred conditions for the conversion of VI to III includereacting a compound of the formula VI wherein Y is defined as above witha C₁ to C₁₀ aliphatic alcohol under conditions which permit dehydrationto form an ester linkage.

Preferred conditions for Step(1)(b) include the same preferred molarratios of reactants as described for Step (1)(a). Also, the preferredreaction temperatures for Step (1)(b) are about the same as for Step(1)(a). Step (1)(b) also proceeds with an actual or formal loss ofwater.

Other preferred conditions for step 1(b) include reacting a compound tothe formula IV with a C₁ to C₁₀ aliphatic alcohol to produce VIII underconditions which permit actual or formal dehydration to form an esterlinkage.

Other preferred conditions for the conversion of VIII to III includereacting a compound of the formula VIII (wherein Q=OR⁴ and R⁴ is a C₁ toC₁₀ straight or branched chain alkyl or aryl group) with a compound ofthe formula V wherein Y and Y' are chosen from the group consisting ofchlorine, bromine, and iodine to produce a compound of the formula III.Optionally, halogen exchange (preferably with iodine) may be effected ona compound of formula III to produce a more reactive compound Also,Compound VIII may be allowed to react with glycine or an appropriatelyprotected and/or activated form of glycine such that a glycinesubstituted dipeptide III is formed wherein Y is either NH₂ or anappropriately substituted nitrogen atom.

The second step ("step 2") of the present invention comprises contactingcompound III with a compound of the formula

    HNWW'                                                      (IX)

wherein W and W' are nitrogen-protecting groups or hydrogen. W may bethe same or different than W'. It is most preferred that W and W' behydrogen. Step 2 is permitted to occur in the presence of an appropriatesolvent, under conditions such that a compound is formed having thefollowing formula ##STR12##

In the case where in Compound III Y is amino or an amino groupsubstituted with one or two nitrogen protecting groups, III can bedirectly converted to X without the need to contact III with IX. In somecases it may be necessary to remove protecting groups (W and/or W') fromIII to form X when Y is an amino group substituted with one or twonitrogen protecting groups.

It is to be understood that various intermediates resulting fromreaction of IX with III may be isolated (for example III where Y=NH₂).These intermediates can then in turn be converted to X.

Appropriate solvents for Step 2 are the same as described for Step (1).A preferred reaction temperature for Step 2 is about -80° to 300° C.with -20° to 40° C. being more preferred. Preferred molar ratios ofcompound IX:compound III are about 10,000:1 to 1:1 with about 1,000:1 to1:1 being more preferred.

Other preferred conditions for Step 2 include reacting a compound of theformula III where Q=OR⁴ where R⁴ is defined as described hereinbeforeand Y=halogen with ammonia in a C₁ to C₁₀ alcohol solvent such asmethanol, ethanol, phenol, benzyl alcohol, n-propanol, etc. Certainreactants, e.g., ammonia, can be used as solvent.

Alternatively, the cyclic dipeptide X may be prepared from an ester of alinear dipeptide containing an appropriate N-terminal amino acid and aC-terminal glycine which is itself prepared by condensation of anN-protected amino acid and an unprotected or carboxy-protected glycine.

The third step ("Step 3") of the present invention comprises reactingcompound X with compounds of the formula ZQ, and Z'Q wherein QH and/orQW' are actually or formally liberated in the reaction, in the presenceof an appropriate solvent and, optionally, at least a catalytic amountof a suitable catalyst, under conditions such that a compound is formedhaving the formula ##STR13## wherein Z, Z', R, and Q are definedhereabove. However, in formula XI it is preferred that Z and Z' be thesame and not hydrogen. It is even more preferred that Z and Z' both beacyl or acyloxy. More preferred is acyl, and most preferred is acetyl.

Appropriate solvents for Step 3 are the same as described for Step 1 andadditionally include an acyl anhydride such as alkyl ##STR14##preferably wherein the alkyl groups contain 1 to 6 carbon atoms;preferred solvents are C₁ to C₂₀ straight chain or branched chaincarboxylic acids, and aprotic solvents such as ethyl acetate, THF, DMF,toluene, or the like. It is preferred that compounds ZQ and Z'Q are thesame. The nature of the Q group in ZQ and/or Z'Q is generally notimportant so long as the Q group permits the incorporation of Z and Z'in the conversion of X to XI. Preferred compounds that can be ZQ and/orZ'Q include acyloxy halides such as benzyl chloroformate, acylanhydrides such as acetic anhydride or enol esters or enol acyloxycompounds such as isopropenyl acetate. It is also preferred that ZQand/or Z'Q act as the reaction's suitable solvent. A large molar excessof ZQ and Z'Q to Compound X is therefore possible, but usually at leastabout 1 mole equivalent of each of ZQ and Z'Q are desired.

Preferred reaction temperatures for Step 3 are about -20° to 300° C.with about 50° to 150° C. being more preferred.

Preferred catalysts for Step 3 are acid catalysts, e.g., strong acid orweak acid catalysts. Such catalysts include Cu^(II) Cl₂ p-toluenesulfonic acid (TsOH), and acetic acid. Preferred catalytic amounts (ormore) include a molar ratio of catalyst:Compound X of about 1:100,000 to10:1; especially preferred for weak acid catalyst is about 1:1,000 to10:1 with about 1:20 to 10:1 being more preferred; especially preferredfor strong acid catalysts is about 1:2000 to 1:100.

In a preferred embodiment of the invention, Z and Z' impart the propertyof crystallinity such that upon crystallization from a suitable solvent,Compound XI can be obtained in wholly or substantially optically pureform. For this crystallization step, a suitable solvent is ethylacetate-heptane mixture. Preferred optical purity after crystallizingCompound XI is greater than about 90%, more preferably greater thanabout 95%, and most preferred is greater than about 99%. For thiscrystallization it is preferred that Z' and Z are the same and are acylor acyloxy, more preferred Z and Z' are acetyl or alpha-halo-acetyl. Thephysical steps for crystallization simply include contacting Compound XIwith solvent such that the desired compound crystallizes, i.e., forms acrystalline solid. The desired compound can then be separated orisolated by conventional techniques such as filtration.

In the case where W'=Z=Z'=H, step 3 is not necessary and in furthersteps Compound X and XI are the same.

The fourth step ("step 4") of the present invention comprises contactingCompound XI with either

a. A compound of the formula

    R'--CHO                                                    (XII)

or

b. a compound of the formula ##STR15## wherein each Q and R',independently, are as defined hereabove, in the presence of an acid orbase and a suitable solvent, under conditions such that a compound isformed having the formula ##STR16## wherein R, R', Z, and Z' are asdefined hereabove and R is the same as R'. Suitable solvents for Step 4are the same as described for Step 1.

Preferred conditions for Step 4(a) include reaction of XI with a strongbase such as potassium t-butoxide (KOtbu), in an aprotic solvent, or ina C₁ to C₁₀ aryl or alkyl or alkylaryl alcohol and permitting reactionof the deprotonated intermediate derived from Compound XI with CompoundXII.

Preferred conditions for Step 4(b) include permitting a deprotonatedintermediate formed as described above to react with compound XIII.

Preferred reaction temperatures for Step 4 are about -80° to 100° C.;more preferred is -20° to 100° C.; most preferred is about -20° to 25°C. Preferred molar ratios of Compound XI to Compound XII or CompoundXIII are about 1:10 to 1:1.

The fifth step ("step 5") of the present invention is the novelhydrogenation step described in the "Summary of the Invention" section.

Suitable solvents for Step 5 are the same as described for Step 1.Preferred solvents for step 5 include acetic acid, DMF, and C₁ to C₁₀alcohols, such as MeOH, EtOH, and iPrOH. Preferred reaction temperaturesfor Step 5 are about -80° to 100° C.; more preferred is about -80° to50° C.; most preferred is about -50° to 25° C. Lower temperatures aregenerally preferred to enhance diastereomeric selectivity in reduction.In Step 5, hydrogen can be hydrogen gas or other source of hydrogen.

Suitable catalysts for Step 5 are common hydrogenation catalysts knownin the art such as transition metal catalysts. Examples includepalladium on carbon (Pd-C), palladium on aluminum, and the like. Molarratio of hydrogen to Compound I are not known to be critical buttypically an excess of hydrogen is used. The amount of catalyst is acatalytic amount or greater; typically a molar ratio ofcatalyst:Compound I of about 1:100,000 to 10:1 is used. A preferredmolar ratio of catalyst:Compound I is about 1:10,000 to 1:1,000.

It is contemplated that when Z and/or Z' are not H, one can optionallytreat I or II to convert Z and/or Z' into H.

The sixth step ("step 6") of the present invention comprises contactingCompound II with an acid under aqueous conditions, such that two molesof compound IV are formed having substantially the same optical rotationas the IV initially used in this process. When Z or Z' are not H in II,they are lost during the production of IV.

In Step 6, water miscible solvents can also be present. A preferredmolar ratio of Compound II:acid is about 1,000:1 to about 1:1,000; morepreferred is about 1:2 to about 1:100. Reaction temperature ispreferably about 20° to 300° C.; preferred is about 50° to 200° C.; andmost preferred is about 100° C. Acids are preferably strong acids suchas HCl, H₂ SO₄, HBr, p-toluene sulfonic acid (TsOH), and the like.

More preferred conditions for step 6 include refluxing a solution orsuspension of Compound II in aqueous hydrochloric acid.

Alternately, Compound II can be treated with acid in a solventcontaining a straight or branched chain alkyl or aryl alcohol to formVIII in high optical purity (i.e., at least about 70%, preferablygreater than about 90%).

It is to be understood that certain compounds described herein can existin salt form. For example, compounds containing an amino moiety,typically readily form acid addition salts such as a hydrochloride,trifluoroacetate, and the like. The salts of such compounds arecontemplated to be within the scope of the invention If a salt form of acompound is present, it may be desirable to optionally convert the saltby simple techniques well known in the art to the free base or salt freeform of the compound.

All of the process steps described herein are preferably carried outunder an inert atmosphere, e.g., under nitrogen or argon. In some cases,the presence of ambient atmosphere is not detrimental.

Preferred processes of the present invention can be represented in theschemes that follow. Rexyn™ is a tradename for a quaternary ammoniumhydroxide anionic exchange resin. The compound numbers can becross-referenced to the examples. ##STR17##

The present invention is illustrated by the following examples; however,such examples should not be interpreted as a limitation upon the scopeof the present invention.

EXAMPLES Example 1 General Procedures

Melting points were determined using a Thomas Hoover capillary meltingpoint apparatus and are uncorrected. Infrared (IR) spectra were recordedon a Perkin-Elmer Model 137 or a Nicolet Model 5DX spectrophotometer andare reported in wave numbers (cm⁻¹). All mass spectra (MS) were obtainedusing a VG Analytical Ltd. Model ZAB-1F Mass Spectrometer in EI(electron impact) and FD (field desorption) modes. GCMS were obtainedusing a Finnigan 4023 GCMS equipped with a 30 m DB5 capillary column (J& W Scientific) using helium carrier gas. Elemental analyses wereperformed by Eastman Chemical Division's Physical and AnalyticalChemistry Research Division using combustion analysis. Optical rotationswere measured using an Autopol III polarimeter manufactured by RudolphResearch.

Unless otherwise specified, all ¹ H NMR spectra were obtained on a JEOLGX-400 NMR instrument operating at 400 MHz. This instrument is capableof a routine resolution of 0.6 Hz.

Chemical shifts are expressed in parts per million relative to internaltetramethylsilane.

High pressure liquid chromatography (HPLC) was accomplished using aVarian 5060 liquid chromatograph equipped with a Zorbax ODS 4.6 mm×25 cmcolumn. Compounds were detected using a Perkin-Elmer LC-75 UV detectorat 254 nm. All injections were a 10 mL volume.

Gas chromatography (gc) was accomplished using a Hewlett Packard 5880Ainstrument in capillary mode using a flame ionization detector unlessotherwise specified. Hydrogen was used as a carrier gas at a flow rateof approximately 40 cm/sec. Unless otherwise specified, a 30 m DB5column (J & W Scientific) was used for GC analyses.

All reactions were carried out under an inert atmosphere of nitrogen orargon unless otherwise specified. Anhydrous tetrahydrofuran (THF) wasprepared by distillation from metallic sodium and benzophenoneimmediately prior to use.

All dimethylformamide (DMF) was distilled using a 1.5×48 in. PodbielniakHelipak column (90 theoretical plates) and only the constant boilingfraction (bp 45° C., 10 mm Hg) was collected. This distilled DMF wasstored in the dark under a nitrogen atmosphere and over 4 A sieves. Ifused within one year, material prepared and stored in this fashion wasfound to contain less than 35 parts per million of water (Karl Fischertitration) and less than 10 parts per million of dimethyl amine asdetermined by cation analysis on a Dionex Model 16 ion chromatograph.The sensitivity of this ion chromatography method for dimethyl aminedetection was found to be approximately 5 parts per million.

Example 2 L-Phenylalanine Methyl Ester Hydrochloride

Thionyl chloride (14 mL, 0.19 mol) was slowly added to a vigorouslystirred -5° C. suspension of L-phenylalanine (26.28 g, 0.159 mol) inmethanol (250 mL). After the addition of thionyl chloride was complete,the resulting homogeneous solution was allowed to warm to roomtemperature and was left stirring overnight. The reaction solvent wasremoved invacuo to produce a white solid. This solid was redissolved inmethanol (100 mL) and the resulting solution was reconcentrated in vacuoto produce the product L-phenylalanine methyl ester hydrochloride (mp158°-159° C., 34.10 g, 0.158 mol, 99.4%). A sample of this material wasconverted to the N-trifluoroacetylisopropyl ester using standardprocedures. The resulting ester was found to contain 0.088% of theD-isomer by gc analysis on a Chirasil-Val™ chiral gc column (obtainedfrom Applied Science). The ¹ H NMR of this synthetic material wasidentical with that of a commercial sample.

¹ H NMR (DMSO-d6): δ=8.62 (bs, 2.1H), 7.35-7.22 (m, 5H), 4.27 (apparentt, J˜6.1, 6.7, 1H), 3.67 (s, 3H), 3.18 (dd, J=6.1, 14.0, 1H), 3.10 (dd,J=7.3, 14.0, 1H)

Optical Rotation: [α]D²⁵ =+37.4° (c=1.95, EtOH)

Analysis: Calc. for ClOH13NO2HCl: C, 55.69; H, 6.54; N, 6.49; Cl, 16.44.

Found: C, 55.70; H, 6.56; N, 6.43; Cl, 16.29

Example 3 N-α-Chloroacetyl-L-phenylalanine Methyl Ester

Chloroacetyl chloride (19 mL, 0.24 mol) was added over a period ofapproximately 5 minutes to a vigorously stirred, 0° C. suspension ofdistilled triethylamine (70 mL, 0.50 mol) and L-phenylalanine methylester hydrochloride (49.95 g, 0.232 mol) in ethyl acetate (400 mL).After a period of 30 minutes at 0°-10° C., the reaction mixture wasfiltered to remove triethylamine hydrochloride. The resulting solutionwas extracted with 1N HCl (2×250 mL), water (250 mL), half-saturatedaqueous sodium carbonate (2×250 mL), and brine (2×200 mL). The organicphase was filtered through magnesium sulfate and sodium sulfate andconcentrated in vacuo to provide the product, 6, as light brown crystals(mp 72.5°-75° C., 55.26 g, 0.216 mol, 93.4%). This material was found tobe of suitable purity (>95% by NMR) for use in subsequent reactions(vide infra). An analytical sample of 6 (mp 76°-77° C.) could beprepared upon crystallization from methanol-water (2/1).

¹ H NMR (CDCl₃): δ=7.33-7.10 (m, 5H), 6.97 (bd, J˜6.1, ¹ H), 4.87 (m,1H), 4.04 (d, J=15.3, 1H), 4.00 (d, J=15.3, 1H), 3.74 (s, 3H), 3.17 (dd,J=6.1, 14.0, 1H), 3.13 (dd, J=6.1, 14.0, 1H)

FDMS: M+=255

IR (KBr): ν=3360, 3175-2860, 1740, 1660, 1550

Example 4 N-α-Iodoacetyl-L-phenylalanine Methyl Ester

N-α-Chloroacetyl-L-phenylalanine methyl ester (6, 10.61 g, 0.0415 mol)was added to a solution of sodium iodide (10.34 g, 0.069 mols) in2-butanone (100 mL). The resulting solution was refluxed for 2 hours and45 minutes. The reaction mixture was filtered to remove precipitatedsodium chloride and the 2-butanone was removed in vacuo. The product wasdissolved in ethyl acetate and extracted with water and brine. Theorganic phase was dried by filtration through magnesium sulfate andsodium sulfate and concentrated in vacuo to provide 7 as a yellow-browncrystalline solid (14.04 g, ˜0.040 mol, ˜98%). This material was notpurified further and was found to be of suitable purity for use in thepreparation of cyclo-glycyl-L-phenylalanine, 8 (vide infra).

¹ H NMR (CDCl₃): δ=7.33-7.12 (m, 5H), 6.48 (bd, J=7.3, 1H), 4.85 (m,1H), 3.75 (s, 3H), 3.69 (d, J=11.6, 1H) 3.65 (d, J=11.6, 1H), 3.17 (dd,J=6.1, 14.0 1H), 3.11 (dd, J=6.1, 14.0, 1H)

FDMS: M+=347

IR (KBr): ν=3330, 3175-2860, 1754, 1667, 1544

Exact mass: Calc. for C₁₂ H₁₄ NO₃ I: 347.0017

Found: 347.0030.

Example 5 N-Benzyloxycarbonyl-glycyl-L-phenylalanine Methyl Ester

Solid carbonyldiimidazole (79.10 g, 0.488 mol) was added to a roomtemperature solution of N-benzyloxycarbonyl-glycine (117.11 g, 0.560mol) in anhydrous tetrahydrofuran (THF, 600 mL). The resulting solutionwas stirred for 45 minutes until CO₂ evolution had ceased. A solution ofL-phenylalanine methyl ester (79.65 g, 0.444 mol) in anhydrous THF (˜200mL) was then added. The reaction temperature was maintained below 40° C.by application of a water bath to the exterior of the reaction vessel.After two hours, a small portion of water (1.5 mL, ˜0.08 mol) was addedto the reaction mixture to hydrolyze any remaining acylimidazolides andthe resulting solution was left to stir overnight. The reaction mixturewas concentrated in vacuo and the residual oil was dissolved in ethylacetate (˜1 L). This ethyl acetate solution was extracted with 1 N HCI(2×300 mL--the first aqueous extract was acidified a pH of 1 by additionof 6 N HCl), water (300 mL), saturated aqueous sodium carbonate (300mL), half-saturated aqueous sodium carbonate (250 mL), and brine (250mL). The organic phase was dried by filtration through magnesium sulfateand sodium sulfate and concentrated in vacuo to constant weight. Fielddesorption mass spectrometry (FDMS) of the resulting oil (160.88 g,˜0.434 mol, 98%) indicated the presence of a single component (M+=370).This material was not further purified, but was instead converteddirectly into cyclo-glycyl-L-phenylalanine (8) by catalytichydrogenation (vide infra).

¹ H NMR (CDCl₃): δ7.34-7.06 (m, 10H), 6.49 (bd, J=6.7, 1H), 5.43 (bs,1H), 5.11 (s, 2H), 4.87 (dd, J=6.1, 6.1, 1H), 3.84 (m, 2H), 3.71 (s,3H), 3.12 (dd, J=6.1, 14.0, 1H), 3.07 (dd, J=6.1, 14.0, 1H)

FDMS: M+=370

IR (film): ν=3330, 3175-2860, 1740, 1680, 1540

Example 6 Cyclo-glycyl-L-phenylalanine, Method A

N-α-Iodoacetyl-L-phenylalanine methyl ester (7, 2.85 g, 0.0082 mol) wasadded to a freshly prepared saturated solution of NH₃ in methanol (145mL) at 18° C. The resulting homogeneous solution was allowed to standovernight at room temperature. The white precipitate of 8 (0.94 g,0.0046 mol, 56%) which had formed overnight was collected by filtration.This material was shown to be of high purity by ¹ H NMR(identical withmaterial prepared by Method B) and optical rotation [α]_(D) ²⁵ =+125°C., c=0.228, CF₃ COOH). The filtrate was concentrated in vacuo toprovide an additional sample of 8 (1.87 g) which was contaminated withinorganic salts. External standard HPLC analysis indicated that thissecond material had a purity of 35%. Thus, the combined yield of 8 inthis reaction was 95%.

Example 7 Cyclo-glycyl-L-phenylalanine (8), Method B

A solution of N-benzyloxycarbonyl-glycyl-L-phenylalanine methyl ester(9, 160.83 g, 0.434 mol) in methanol (500 mL) was added to a vigorouslystirred suspension of 5% palladium on carbon (11.4 g) in methanol (1 L).Argon was bubbled through the reaction mixture for 15 minutes. Hydrogenwas then introduced into the reaction vessel at a rate such that a slowflow of hydrogen gas (1 atm) was maintained through the vessel. Beforecompletion of the reaction, it was necessary to apply a cooling bath toprevent loss of methanol due to the mildly exothermic nature of thereaction. After a period of three hours (hydrogen uptake had ceasedafter two hours), the reaction vessel was purged with argon and thereaction mixture was filtered through celite. Additional methanol wasadded to the filtrate to bring the final volume to 3.5 litres. Thissolution was set aside at room temperature for a period of one week.Filtration of the resulting precipitate provided the product 8 (mp267°-269° C., dec; [ ]_(D) ²⁵ =+133.8°, c=0.204, CF₃ COOH) as a whitesolid (66.76 g, 0.327 mole, 75%).

This second filtrate was set aside and after a period of 2-3 weeks allprecipitation appeared to cease. Filtration of the precipitate providedan additional crop of 8 (4.57 g, 5.1%).

An analytical sample of 8 (mp 269.5°-270.5° C., dec. was prepared byrecrystallization of the first filtrate from hot methanol ([α]_(D) ²⁵=+133.2°, c=0.202, CF₃ COOH).

¹ H NMR (DMSO-d₆): δ=8.16 (bs, 1H), 7.89 (bs, 1H), 7.30-7.16 (m, 5H),4.07 (m, 1H), 3.34 (d, J=17.7, 1H), 3.09 (dd, J=4.3, 13.4, 1H), 2.88(dd, J=4.9, 13.4, 1H), 2.76 (d, J=17.7, 1H)

FDMS: M+=204

IR (KBr): ν=3330-2860, 1680, 1470

Analysis: Calc. for C₁₁ H₁₂ N₂ O₂ : C, 64.69; H, 5.92; N, 13.72

Found: C, 64.62; H, 5.77; N, 13.51

Example 8 N,N'-Diacetyl-cyclo-glycyl-L-phenylalanine (10)

A stirred suspension of cyclo-glycyl-L-phenylalanine (8, 41.03 g, 0.201mol) in acetic anhydride (390 mL) was heated to reflux. In approximately20 minutes, the reaction mixture became homogeneous. After a period of 3hours and 40 minutes at reflux, the now pale yellow reaction mixture wascooled and the solvent was removed in vacuo. The viscous oilcrystallized when seeded with a previously prepared sample of 10. Exceptfor traces of what appeared to be acetic anhydride and acetic acid, thiscrude crystalline product (62.3 g, 107%) was shown to be a singlecomponent by ¹ H NMR. An analytical sample of 10 (mp 102°-103° C.,45.81g, 0.159 mol, 80%) was prepared upon crystallization of thismaterial (61.67 g) from hot ethyl acetate (100 ml) and heptane (1.1 L).The residual liquid from this crystallization was concentrated in vacuoand the resulting pale yellow crystals were dried under high vacuum (0.5mm, 35° C., 18 hours) to constant weight (11.49 g, 0.40 mol, 20%). Bothof these samples obtained from this crystallization were shown to beidentical by ¹ H NMR.

The optical purity of the analytical sample ([α]D²³ ˜+61.56°, c=0.307,EtOH; [α]D²⁵ =+79.6, c=0.353, EtOAc) was found to be greater than 98%when analyzed by 400 MHz NMR (CDCl₃) in the presence of the chiral shiftreagent (-)2,2,2-trifluoro-1-(9-anthryl)-ethanol. However, when analyzedby this same method, the material isolated from the mother liquors wasfound to contain between 15 and 25% of the D isomer.

In a repetition of this experiment (10 g scale), an 86% yield ofoptically pure recrystallized product was obtained.

¹ H NMR (CDCl₃): δ=7.35-7.05 (m, 5H), 5.45 (apparent t, J˜4.9, 1H), 4.49(d, J=18.9, 1H), 3.35 (dd, J=4.3, 14.0, 1H), 3.21 (dd, J=5.5, 14.0 1H),2.59 (s, 3H), 2.56 (s, 3H), 2.46 (d, J=18.9, 1H)

FDMS: M+=288

IR (KBr): ν=3175-2860, 1724

Analysis: Calc. for C₁₅ H₁₆ N₂ O₄ : C, 62.49; H, 5.59; N, 9.72

Found: C, 62.17, H, 5.51; N, 9.49

Example 9 (S)-1-Acetyl-3-benzylidene-6-benzyl-2,5-piperazinedione

A freshly prepared solution of potassium t-butoxide in t-butyl alcohol(˜0.88 M, 16.9 mL, 0.015 mol) was slowly added over a 45-minute periodto a 5°-10° C. solution of the recrystallized diketopiperazine 10 (4.27g, 0.0148 mol) and benzaldehyde (3.0 mL, 0.030 mol) in distilledanhydrous dimethylformamide (15 mL). After the addition of base wascompleted, a very thick suspension of salts had resulted. Thissuspension was stirred at 10° C. for 30 minutes and then for 2.5 hoursat room temperature. The solvent was removed in vacuo and the reactionmixture was dissolved in ethyl acetate (100 mL). This solution wasfiltered to remove a small amount of a white precipitate and was thenextracted with water (100 mL). The organic phase was dried overmagnesium sulfate and concentrated in vacuo to provide the product 11(5.21 g, 105%) as a viscous oil which was shown to contain traces ofethyl acetate (˜6 mole %) and benzaldehyde (˜20 mole %) by ¹ H NMR. Thismaterial was not purified further, but was instead converted to theinsoluble desacetyl diketopiperazine 3 (vide infra). ¹ H NMR analysisusing the chiral shift reagent (-)-2,2,2,-trifluoro-1-(9-anthryl)ethanolin CDCl₃ indicated that there was 1% or less of the D-isomer in 11 whenprepared by the above method.

¹ H NMR (CDCl₃); α=7.75 (bs, 1H), 7.41-7.09 (m, 10H), 6.61 (s, 1H), 5.37(apparent t, J˜4.5, 1H), 3.30 (dd, J=4.3, 14.0, 1H), 3.25 (dd, J=5.5,14.0, 1H), 2.62 (s, 3H)

FDMS: M+=334

Exact mass: Calc. for C₂₀ H₁₈ N₂ O₃ :334.1313

Found: 334.1325

Optical Rotation: [α]D²⁵ =-456° (c=0.89, EtOH);

Example 10 (S)-3-Benzylidene-6-benzyl-2,5-piperazinedione

Hydrazine hydrate (1 mL, ˜0.021 mol) was added to a vigorously stirredsolution of diketopiperazine 11 (4.67 g, ˜0.014 mol) in DMF (30 mL).There occurred an almost instantaneous precipitation of the product as awhite solid. The reaction mixture was concentrated in vacuo andtriturated with cold water (3×7 mL). The product (3.74 g, ˜0.013 mol,˜92%) was dried to constant weight in vacuo and was shown to be of highpurity by ¹ H NMR (with the exception of ˜20 mole % of DMF). Ananalytical sample of 3 (mp 288°-289° C., was prepared byrecrystallization of the precipitate from hot acetic acid-methanol (1/3,v/v).

¹ NMR (DMSO-d₆): δ=9.71 (bs, 1H), 8.45 (bs, 1H), 7.35-7.12 (m, 10H),6.33 (s, 1H), 4.34 (bs, 1H), 3.15 (dd, J=4.3, 13.4, 1H), 2.97 (dd,J=4.9, 13.4, 1H)

FDMS: M+=292

IR (KBr): ν=3204, 3062, 1671, 1632, 1499, 1454, 1435, 1402, 1350, 1313,1206, 1194, 1094, 924, 910, 874, 860, 822, 738, 638

Analysis: Calc. for C₁₈ H₁₆ N₂ O₂ : C, 73.96; H, 5.52; N, 9.58

Found: C, 73.79; H, 5.40; N, 9.42

UV (95%) ethanol): λmax=295 nm; log ε=4.295

Optical Rotation: [α]D²⁵ =-542° (c=0.0306, HOAc);

Example 11 Cyclo-L-phenylalanyl-L-phenylalanine

Palladium black (0.1 g) was added to a vigorously stirred suspension(under argon) of the dehydro-diketopiprazine 3 (0.73 g, 0.0025 mol) inDMF (90 mL). Hydrogen (1 atm) was then introduced into the reactionvessel. After a period of three days, gas chromatographic analysis (gc)of the reaction mixture indicated incomplete reaction. A second portionof palladium black (0.2 g) was added to the reaction. The reactionsuspension was stirred for an additional four days under a hydrogenatmosphere. The reaction product was dissolved in 250 mL of hot DMF andfiltered to remove catalyst. Concentration in vacuo provided 12 as awhite solid (0.63 g, 0.0022 mol, 85%). HPLC analysis of this sampleindicated that the ratio of cis- to trans-isomer in this sample was99/1. Except for the presence of approximately 3% of thedehydrodiketo-piperazine 3, the 400 MHz 1H NMR of this material wasidentical with a sample of 11 which had been prepared previously by anindependent method of synthesis. A sample of the reduction product 12(prepared as described above) was hydrolyzed in refluxing 6N HCl (20 hr)and converted into the N-trifluoroacetyl-isopropyl ester ofphenylalanine by standard procedures. The ratio of D to L isomer asdetermined by gc analysis on a Chirasil-Val™ capillary gc column wasfound to be 4.2/95.8.

Example 12 DL- 1-Acetyl-3,6-cis-dibenzyl-2,5-piperazine-dione (DL-13)

Hydrogen (1 atm) was introduced into a reaction vessel containing 5%palladium on carbon (0.1 g), the DL-N-acetyl-diketopiperazine 11 (1.07g, 0.0032 mol) and methanol (50 mL). After a period of 22 hours, thereaction was filtered through a Milex-HV 0.45 μm filter unit andconcentrated in vacuo to provide the product DL-13 as a viscous oil(0.92 g, 0.0027 mol, 85%). In addition to indicating a high level ofpurity, ¹ H NMR indicated the presence of the cis-isomer as the majorcomponent (the ratio of cis to trans-isomer was estimated to beapproximately 12 to 1 or greater). Further evidence for the presence ofa major cis- and a minor trans-isomer was obtained from the gc/ms whichindicated the presence of two isomeric components (MWT 336) in a ratioof 13/1.

¹ H NMR (CDCl₃ -D₂ O): δ=7.42-7.12 (m, 10H), 5.19 (apparent t, J=3.7,4.9 1H) 4.02 (dd, J=3.0, 11.6, 1H), 3.34 (dd, J=3.7, 14.0, 1H), 3.23(dd, J=4.9, 14.0, 1H), 2.87 (dd, J=3.0, 13.5, 1H), 2.65 (s, 3H), 0.80(dd, J=11.6, 13.5, 1H)

Example 13 Preparation of L-Phenylalanine Methyl Ester (5b) FromCyclo-L-phenylalanyl-L-phenylalanine (12)

Cyclo-L-phenylalanyl-L-phenylalanine (12, 1.73 g, 0.0059 mol) was addedto a 3M solution of concentrated sulfuric acid in methanol (50 mL). Thesolution was brought to reflux (66° C.) and the initially heterogeneoussolution became homogeneous in several hours (intermediate formation ofthe sulfate salt of L-phenylalanyl-L-phenylalanine methyl ester). Theprogress of the reaction was periodically monitored by gc (afteraddition to saturated sodium carbonate and extraction with ethylacetate). Approximately 6 days at reflux was required to convert 50% ofthe reaction mixture to L-phenylalanine methyl ester. After refluxingfor a period of 17 days, the reaction mixture was cooled to roomtemperature and added to an ice-cold mixture of saturated aqueous sodiumcarbonate (250 mL), 2N sodium hydroxide (50 mL), and ethyl acetate (250mL). The organic phase was removed and the aqueous phase (pH 11) wasrepeatedly extracted with ethyl acetate (3×250 mL). The combined organicphases were dried by filtration through sodium sulfate and concentratedin vacuo to provide ester 5b (1.79 g, 0.010 mol, 85%) as a pale yellowoil. The identity and chemical purity of the L-phenylalanine methylester prepared by this method was established by comparison of the ¹ HNMR for this material with the ¹ H NMR for a sample of ester 5b obtainedby an independent method. The D-isomer content for ester 5b whenprepared by the above method was determined to be 5.5% by gc analysis(after derivatization to the N-trifluoroacetyl isopropyl ester) on aChirasil-Val™ capillary column.

Example 14 Preparation of L-Phenylalanine Hydrochloride (14) FromCyclo-L-phenylalanyl-L-phenylalanine

Cyclo-L-phenylalanyl-L-phenylalanine (12, 1.0818 g, 0.0036 mol) wasadded to a solution of 6N HCl (40 mL). The reaction mixture was broughtto reflux and over a period of 8 hours the initially heterogeneoussolution turned homogeneous. After a total of 24 hours at reflux, thereaction mixture was concentrated in vacuo. An unsuccessful attempt wasmade to convert the hydrochloride product 14 to L-phenylalanine byredissolving the crude product in distilled water (6×30 mL) followed byremoval of the solvent under high vacuum (0.5 mm, 45° C.). The resultingmaterial (1.4337 g, ˜0.0036 mol, ˜100%) was dried to constant weight andwas verified to be phenylalanine hydrochloride by elemental analysis.This material was found to contain 1.9% of the D-isomer by analysis(after derivatization to the N-trifluoroacetyl isopropyl ester) on aChirasil-Val capillary gc column.

¹ H NMR (D₂ O): δ=7.44-7 32 (m, 5H), 3.99 (dd, J=5.5, 7.9, 1H), 3.29(dd, J=5.5, 14.6, 1H), 3.13 (dd, J=7.9, 14.6, 1H)

FDMS: 166 (--Cl)

Analysis: Calc. for C₉ H₁₂ NO₂ Cl: C, 53.61; H, 5.60; N, 6.95; Cl, 17.58

Found C, 53.29; H, 5.96; N, 6.85; Cl, 16.9

We claim:
 1. A compound of the formula ##STR18## wherein R is alkyl,hydroxy, C₁ to C₁₀ alkoxy C₁ to C₁₀ carboxyalkyl, C₁ to C₁₀ acyloxy,phenyl optionally substituted with one to four moieties chosen from thegroup consisting of halogen, hydroxy, protected hydroxy, cyano, nitro,C₁ to C₄ alkyl, C₁ to C₄ alkoxy, carboxy, carboxymethyl, hydroxymethyl,aminomethyl, trifluoromethyl and n-(methylsulfonylamino) or phenyl(C₁ toC₆)alkyl wherein the alkyl portion is optionally substituted with one ortwo moieties chosen from the group consisting of halogen, hydroxy, C₁ toC₇ acyloxy, nitro carboxy, carbamoyl, carbamoyloxy, cyano,N-(methylsulfonylamino), and C₁ to C₄ alkoxy, and the phenyl portion isoptionally substituted with one or two moieties chosen from the groupconsisting of halogen, hydroxy, nitro, C₁ to C₄ alkyl, C₁ to C₄ alkoxy,carboxy, carboxymethyl, hydroxymethyl, aminomethyl, andN-(methylsulfonylamino); R' is the same as R: and Z is anitrogen-protecting group.
 2. The compound of claim 1 wherein Z is acylor acyloxy and both R and R' are the same and are said phenyl,optionally substituted.
 3. The compound of claim 2 wherein Z is acetylor alpha-halo-acetyl.
 4. A compound of the formula ##STR19## wherein Acis acetyl.
 5. The compound of claim 1 wherein the diastereomeric andoptical purity of the compound are at least 90% of the cis derivative,and wherein both asymmetric carbon atoms are substantially of both D orboth L configuration.
 6. The compound of claim 5 wherein thediastereomeric and optical purity of the compound are at least 95% ofthe cis derivative
 7. The compound of claim 3 wherein R and R' arephenyl.
 8. The compound of claim 4 wherein the diastereomeric andoptical purity of the compound are at least 90% of the cis derivative,and wherein both asymmetric carbon atoms are substantially of both D orboth L configuration.
 9. The compound of claim 8 wherein thediastereomeric and optical purity of the compound are at least 95% ofthe cis derivative.